A major source of molybdenum metal is the mineral molybdenite (MoS.sub.2) which occurs frequently as a constituent of other metal sulfide ores or may occur as a primary molybdenite ore with only minor amounts of other metal sulfides. Frequently, molybdenite is a minor constituent of copper sulfide ores, and a major source of molybdenite in the United States is as a by-product of copper ore processing in which the molybdenite is separated from the copper and other sulfide constituents by a froth flotation procedure. Molybdenite is also obtained from primary molybdenite ores by use of a series of froth flotation procedures to obtain a concentrate high in molybdenum sulfide but containing minor amounts of copper sulfide contaminant. Such copper is undesirable since the molybdenite concentrate is usually converted to molybdenum oxide or ferromolybdenum for use by the iron and steel industry which requires the copper content to be low, generally less than 1%.
According to the present practice of the industry, undesirable metal sulfides such as copper and iron sulfide are controlled by use of sodium cyanide or Nokes reagent as a depressant in the froth flotation procedure. However, the toxic nature of the cyanide makes it unattractive because of its potential adverse environmetal effects. Nokes reagent, which is produced by reaction of phosphorous pentasulfide with caustic soda also presents problems since toxic hydrogen sulfide gas is produced as a by-product. The present invention provides a novel combination of reagents which depress copper and other metal sulfides in the froth flotation of molybdenite without the above undesirable environmental side effects.